The drying section of a paper machine comprises generally two tiers of drying cylinders, through which the web is passed, guided by a support fabric which carries it, the web and the support fabric running along a tortuous part alternately between the cylinders of the first tier and the second tier. At the drying cylinders of the first tier the web lies against the circumferential surface of the cylinder and the support fabric is situated on top of it, and at the cylinders of the second tier the web lies outermost while the support fabric lies against the circumferential surface. Consequently, at the location where the web leaves a cylinder of the first tier, and at the location where it enters a cylinder of the second tier, a wedge-like space is formed, in which due to the movements of the cylinders and the support fabric some phenomena causing problems in the run of the web are present. These phenomena are of greater importance as the work speeds of the paper machines have increased. At the location where the web leaves the cylinder of the first tier a negative pressure develops in the wedge-like space situated adjacent to the first cylinder. The subpressure tends to lift the web out of contact with the support fabric. Furthermore, a positive pressure develops in the wedge-like space located at the joining point between the cylinder of the second tier and the support fabric on the opposite side of the web and the support fabric. The positive pressure tends further to keep the web separated from the support fabric wrapping round the second cylinder. This causes edge flutter and torsion in the paper web and creates risks of tearing and wrinkling of the web which is being dried.
It is common to attempt to counteract the above mentioned harmful positive and negative pressures with the help of boxes provided along the free run portion of the web and the support fabric between a first tier cylinder and a second tier cylinder. The aim is to guide the air by means of air streams issuing from the boxes so that the air streams so generated would be opposite to the air streams that give rise to the positive and negative pressures. The boxes of this type are usually positioned on the side of the support fabric at the free run portion so that they can have an influence both on the wedge-like space situated on the other side of the support fabric and on the wedge-like space situated on the same side of the support fabric. The blow boxes of the above mentioned type have been disclosed, for example, in Finnish Patent No. 72547 and in International Patent Application PCT/US86/00745 (Publication No. WO 87/06283). The above mentioned boxes comprise as a rule a chamber connected to a source of pressurized medium, such as air, as well as a nozzle slot in the wall of the chamber for producing the air stream affecting the positive and negative pressures within the wedge-like spaces. A common drawback of the blowing boxes shown in the above mentioned publications and of other well-known blow boxes is that the nozzle slots are directed either against the directions of movements of the support fabric or against the circumferential surfaces of the cylinders. Hence, great blowing efficiency is necessary in order to produce a satisfactory result.